1. Field of the Invention
The present invention is related to a high quality visual display system with high performance and low power consumption. In particular, the present invention is related to a visual display system using multiple image sources, each of which independently produces an image with distinct information content. The images are subsequently or simultaneously combined together in an optical system to form a final superimposed image. More particularly, the present invention is related to see-through visual display systems with very high image brightness, such as heads-up-display systems.
2. Description of the Related Art
Most visual display systems include a single image source. Some visual display systems include multiple image sources but usually each image source displays the same information content. The Cathode-ray-tube (CRT) has been the primary display technology for years, and is used in many types of visual display systems. A CRT can produce high quality and high brightness images with good efficiency. Small size CRT displays with proper phosphors can produce a high quality image with luminance of several thousand to more than ten thousand foot-Lamberts. In addition, CRTs can operate in both raster scan mode, a sequential addressing scheme for producing graphic images; and, stroke mode, a random addressing scheme for producing simple symbolic images. Stroke mode is also known as vector scan mode. It addresses the electron beam directly to the phosphor screen according to the x-y coordinate information, in contrast to the line-by-line scan of a raster scan scheme. In the stroke mode or vector scan mode, sometimes also referred as the vector drawing mode, only simple symbolic or iconic images are presented in designated locations on the phosphor screen. The scanning speed can be relatively slower in comparison to the raster scan, thus producing much higher luminance than the images produced by the raster scan mode. Typical symbology images contain much less addressed pixels, so they can be generated within a relatively short period of time. This is critical for time-sensitive applications. CRT displays also have very high static and dynamic contrast ratios, because the electron beam is turned-off when scanning through the dark field or the background of symbolic images.
However, CRT displays have certain shortcomings. First, they have relatively low reliability because of their high operating voltage. For applications that require high reliability under harsh environmental conditions, CRT displays require frequent and expensive maintenance. Second, CRT displays have a limit on resolution, because of their magnetic-deflecting mechanisms. This imposes not only a limit on the resolution of the image, but also restricts the maximum information content to be displayed. This is particularly important for displaying high quality graphic or video images. Compared to other display products of the same size, CRT displays are relatively bulky and heavy, not to mention the volume and weight of their high voltage power supplies. When a CRT operates in the dual mode, raster mode and stroke mode in a time sequential manner, to display high brightness symbology and relatively low brightness video images together, both raster video resolution and the symbology content are limited. In addition, the analog nature of CRT displays has prevented them from being implemented in many digital applications. Because of all of these disadvantages, CRT displays have been replaced by more reliable, high resolution, and all digital display technology, such as active-matrix-liquid crystal displays (AMLCD) in the market that was once dominated by CRTs, such as television sets, computer monitors, and displays in the cockpits of commercial and military aircraft. Today, the availability of CRT products is from scarce to none.
Liquid crystal displays have evolved to become the dominant display technology in the last 20 years, replacing CRT displays in many applications, including computer monitors and television sets. The primary liquid crystal display technology today is the active-matrix addressed liquid crystal light valves in a variety of sizes, resolutions, and optical schemes. Liquid crystal displays are light in weight, compared to CRT displays of the same size. They have acceptable wall-plug efficiency, as a result of the high efficiency backlight; can be made at very high resolution; and they can operate reliably under harsh conditions. Although still relatively expensive, LCD technology is the choice of the display technology for today's digital information display systems.
However, liquid crystal displays, although improved dramatically over last ten years, still suffer by relatively poor optical performance. There are two prominent performance issues associated with liquid crystal displays: 1) Low contrast due to light leakage from the light valve of the OFF pixels; and, 2) Motion artifact due to the slow electro-optical response of liquid crystal material, and the addressing scheme. Low contrast ratio severely reduces the dynamic range of the image, and causes the dark background of the image to “glow”. This is particularly problematic when viewing the display in dark ambient. The background glow can be very disturbing and distracting. The motion artifact not only degrades the overall quality of the image, but also may cause important information on the image to be blurred, distorted, delayed, or missed. Typical motion artifact is the “contrail” behind a fast-moving subject in the image.
In the active matrix liquid crystal displays (AMLCD), the image pixels are controlled by the actively addressed array of thin-film-transistors (TFT) using the sequential addressing, a raster scan scheme, to generate the image in a line-by-line fashion. Different than CRTs, AMLCDs can only operate in the raster mode. For displaying typical graphic or video images, this is adequate enough. However, in some special display systems, such as heads-up-displays, where symbology image is the dominant content of the information, and often changing rapidly, the raster scan type addressing scheme shows significant latency in displaying the information.
The efficiency of liquid crystal displays is relatively low. It is a function of pixel size. For large format displays, in which the pixels are relatively large, the efficiency does not seem to be an issue. However, small size and high resolution AMLCDs have fairly low transmission efficiency, typically only ˜5% or less for full color displays, and <15% for monochrome displays. For typical computer monitors or TV sets, which require a few tens to 100-300 foot-Lambert luminance, such a low efficiency is not necessarily a problem. However, at such a low efficiency, it would require extremely bright backlight to produce display luminance higher than 1,000 foot-Lambert. To produce 10,000 foot-Lambert, the backlight design becomes a scientific and engineering challenge. Practically, a high resolution small AMLCD display with luminance at 10,000 foot-Lambert will consume more power than an equal size CRT. The volume of such as display, including the cooling mechanism for the backlight, is not much smaller than the CRT. Therefore, liquid crystal displays are the best at producing an image with luminance below ˜1000 foot-Lambert.
Optical scanning is another display technology. In recent years, miniature micro-electro-mechanical system (MEMS)-based one-dimensional or two-dimensional optical scanners are emerging to become a viable display technology. Optical scanners steer a high intensity light beam into a designated direction and location. Combined with the modulation of the light beam, an optical scanner can produce an image on an image screen, similar to a CRT display, which scans an electron beam to form an image on a phosphor screen. Optical scanner-based displays function like emissive displays, even though the scanner device themselves are non-emissive. Light source directly modulated by the image data signal can produce very high display contrast. These miniature optical scanners are compact in size; use a minimum amount of power; have very high optical efficiency; and, can scan at very high speed. Display systems using these optical scanners have been demonstrated. Two-dimensional images can be generated by two one-dimensional scanners or one two-dimensional scanner.
Optical scanners, depending on scanner design and implementation, can operate 1) in vector drawing mode, a random addressing scheme similar to stroke mode in a CRT, drawing the image according to the coordinate information of the light beam; or, 2) raster scan mode, scanning the light beam onto an image screen in a line-by-line fashion. Vector scanners typically consist of two one-dimensional scanners: a first one directing light in a horizontal direction and a second one directing light from the first one in vertical direction, or vise versa. This type of optical scanner is suitable for producing symbology images with very high efficiency and high resolution.
Optical scanners can also be used to produce graphic or video images. Either one-dimensional or two-dimensional devices can be used in generating video images. In either case, the scanners operate in the raster mode, scanning the light beam line-by-line to form the image. One-dimensional video scanners have been used to produce high quality images, such as in specialty large format laser scanning displays. Two dimensional MEMS scanners have been developed to show video or graphic images. The video display systems using one-dimensional scanners are relatively complex in optical and mechanical design. The video displays using two-dimensional MEMS devices have limited resolution because of the limit of the resonance frequency of such devices.
There are other types of light modulating devices that can produce high quality images. For example, the Spatial Light Modulators or other Optical Phase Array (OPA) devices can produce images using the phase modulation/interference of the light beam. In principle, these type of devices can generate high contrast image with minimum power consumption. They all have been proposed or already implemented in some visual display systems.
For display systems requiring luminance up to 10,000 foot-Lambert, CRT and optical scanners seem to be the ideal choice for a high efficiency and low power consumption system to produce symbology images using a vector scan addressing scheme. For high resolution graphic or video displays, active matrix addressed liquid crystal displays are good choices, but image luminance is limited to 1,000-2,000 foot-Lambert maximum because of the limit in backlight technology, and restrains of power of the display system.
Small to medium size CRTs have previously and are still being used as the image source for heads-up-displays (HUD) in the last several decades. CRTs have some ideal characteristics for HUD applications, such as extremely high luminance, low latency, high dynamic range, high contrast ratio, good image quality, relatively low power consumption, and they can function under harsh ambient conditions. Not until recent years have AMLCD image sources been implemented in some new HUD designs. However, AMLCDs cannot simply replace CRT image sources in many existing HUD systems, because they lack some of the critical characteristics of CRTs, such as high contrast ratio and high luminance at low power consumption. Optical image scanners are a new emerging display technology, having some similar characteristics as CRT displays, such as high contrast ratio, high luminance, compact size and low power consumption, and functionality over wide ambient conditions. Especially, the vector scanning type devices have been demonstrated to produce high luminance and high quality symbology images. It appears that optical scanners can become a feasible candidate for the image source in a HUD system for producing high luminance symbology and graphic images.
In a HUD system in which both graphic/video and symbology images are displayed, the graphic/video images and symbology images are often from different origins but combined into a data stream as the input to the HUD system. For instance, the raster video image may be from a camera or from other types of image sensors; and, the stroke symbology images are from a graphic generation engine of the instrument or computer system of a vehicle on which the HUD is installed. The image signals of both types of images are combined to be sent through a common input interface to the image source, and then displayed in a time-sequential manner in a CRT display. However, to display such a combined image signal on an AMLCD, the signal has to be converted to a raster-only signal for the AMLCD to display. Combining and later converting the symbology and video data may result in significant delays in displaying time-sensitive information to the viewer of the HUD. It would be ideal if different types of images from different origins can be displayed simultaneously or sequentially in an image producing system without any combination and subsequent conversion or separation of image signals.